The present invention relates to an endpoint detector and a method for quickly and accurately measuring a change in thickness of a semiconductor wafer or another microelectronic substrate in mechanical or chemical-mechanical polishing processes.
Chemical-mechanical polishing or mechanical polishing processes (collectively xe2x80x9cCMPxe2x80x9d) remove material from the surface of a microelectronic substrate (e.g., a semiconductor wafer) in the production of ultra-high density integrated circuits. In a typical CMP process, a wafer is pressed against a planarizing medium (e.g., a polishing pad) in the presence of a planarizing fluid (e.g., an abrasive slurry) under controlled chemical, pressure, velocity, and temperature conditions. The planarizing fluid may contain small, abrasive particles to abrade the surface of the wafer, but a non-abrasive planarizing fluid may be used with fixed-abrasive polishing pads. Additionally, the planarizing fluid has chemicals that etch and/or oxidize the surface of the wafer. The polishing pad is generally a planar pad made from a porous material, such as blown polyurethane, and it may also have abrasive particles bonded to the material. Thus, when the pad and/or the wafer moves with respect to the other, material is removed from the surface of the wafer by the abrasive particles (mechanical removal) and the chemicals (chemical removal).
FIG. 1 schematically illustrates a conventional CMP machine 10 with a platen 20, a wafer carrier 30, a polishing pad 40, and a slurry 44 on the polishing pad. The platen 20 has a surface 22 upon which the polishing pad 40 is positioned. A drive assembly 26 rotates the platen 20 as indicated by arrow xe2x80x9cAxe2x80x9d. In another type of existing CMP machine, the drive assembly 26 reciprocates the platen back and forth as indicated by arrow xe2x80x9cBxe2x80x9d. The motion of the platen 20 is imparted to the pad 40 because the polishing pad 40 frictionally engages the surface 22 of the platen 20. The wafer carrier 30 has a lower surface 32 to which a wafer 60 may be attached, or the wafer 60 may be attached to a resilient pad 34 positioned between the wafer 60 and the lower surface 32. The wafer carrier 30 may be a weighted, free-floating wafer carrier, or an actuator assembly 36 may be attached to the wafer carrier 30 to impart axial and rotational motion, as indicated by arrows xe2x80x9cCxe2x80x9d and xe2x80x9cDxe2x80x9d, respectively.
In the operation of the conventional polisher 10, the wafer 60 is positioned face-downward against the polishing pad 40, and then the platen 20 and the wafer carrier 30 move relative to one another. As the face of the wafer 60 moves across the planarizing surface 42 of the polishing pad 40, the polishing pad 40 and the slurry 44 remove material from the wafer 60.
In the competitive semiconductor industry, it is highly desirable to maximize the throughput of CMP processes to produce accurate, planar surfaces as quickly as possible. The throughput of CMP processes is a function of several factors, one of which is the ability to accurately stop the CMP process at a desired endpoint. Accurately stopping the CMP process at a desired endpoint is important to maintaining a high throughput because the thickness of the dielectric layer must be within an acceptable range; if the thickness of the dielectric layer is not within an acceptable range, the wafer must be re-polished until it reaches the desired endpoint. Re-polishing a wafer, however, significantly reduces the throughput of CMP processes. Thus, it is highly desirable to stop the CMP process at the desired endpoint.
In one conventional method for determining the endpoint of the CMP process, the polishing period of one wafer in a run is estimated using the polishing rate of previous wafers in the run. The estimated polishing period for the wafer, however, may not be accurate because the polishing rate may change from one wafer to another. Thus, this method may not accurately polish the wafer to the desired endpoint.
In another method for determining the endpoint of the CMP process, the wafer is removed from the pad and wafer carrier, and then the thickness of the wafer is measured. Removing the wafer from the pad and wafer carrier, however, is time-consuming and may damage the wafer. Moreover, if the wafer is not at the desired endpoint, then even more time is required to re-mount the wafer to the wafer carrier for repolishing. Thus, this method generally reduces the throughput of the CMP process.
In still another method for determining the endpoint of the CMP process, a portion of the wafer is moved beyond the edge of the pad, and an interferometer directs a beam of light directly onto the exposed portion of the wafer. The wafer, however, may not be in the same reference position each time it overhangs the pad because the edge of the pad is compressible, the wafer may pivot when it overhangs the pad, and the exposed portion of the wafer may vary from one measurement to the next. Thus, this method may inaccurately measure the change in thickness of the wafer.
In light of the problems with conventional endpoint detection techniques, it would be desirable to develop an apparatus and a method for quickly and accurately measuring the change in thickness of a wafer during the CMP process.
In addition to accurately determining the endpoint of CMP processes, it is also desirable to monitor other performance characteristics or parameters to maintain the throughput and quality of finished wafers. The performance of CMP processes may be affected by the pad condition, the distribution of planarizing fluid under the wafer, and many other planarizing parameters. Monitoring these parameters, however, is difficult because it is time consuming to interrupt processing wafers to determine whether one of the parameters has changed. Moreover, if the CMP process is stopped and all of the parameters appear to be in an acceptable range, it is a complete waste of processing time. Thus, it would also be desirable to monitor the performance of CMP processing to ensure that the planarizing parameters are within acceptable operating ranges without interrupting the process.
The invention is directed, in part, to detecting the endpoint of a planarization process that removes material from a microelectronic substrate, such as a semiconductor wafer. An endpoint detector measures the change in thickness of a semiconductor wafer while the wafer is attached to a wafer carrier during chemical-mechanical polishing of the wafer. The endpoint detector has a reference platform, a measuring face, and a distance measuring device. The reference platform is positioned proximate to the wafer carrier, and the reference platform and measuring device are positioned apart from one another by a known, constant distance for all of the measurements of a single wafer. The measuring face is fixedly positioned with respect to the wafer carrier at a location that allows the measuring device to engage the measuring face when the wafer is positioned on the reference platform. Each time the measuring device engages the measuring surface, it measures the displacement of the measuring face with respect to the measuring device. The displacement of the measuring face is proportional to the change in thickness of the wafer between measurements.
In a method in accordance with the invention, the wafer is placed on the reference platform before it is polished, and then the measuring device engages the measuring surface to determine a baseline measurement of the position of the measuring face with respect to the measuring device. After, the wafer is at least partially polished, the wafer is re-placed on the reference platform and the measuring device is re-engaged with the measuring face to determine a subsequent measurement of the position of the measuring face with respect to the measuring device. The displacement of the measuring face from the baseline measurement to the subsequent measurement is proportionate to the change in thickness of the wafer.
The invention is also directed towards planarizing machines and methods for monitoring the polishing rate, endpoint, and other planarizing parameters of CMP processes. A planarizing machine in accordance with the invention includes a flat plate, a planarizing medium fastened to the plate, a carrier assembly to manipulate a substrate with respect to the planarizing medium, and a non-contact distance measuring device. The carrier assembly, more specifically, may have a support structure and a substrate holder coupled to the support structure. The substrate holder typically has a mounting site facing the planarizing medium to carry the substrate, and a backside facing away from the planarizing medium. The non-contact distance measuring device may also be attached to the support structure to be positioned over the substrate holder for at least a portion of the planarizing process. Additionally, the support structure typically holds the non-contact measuring device at a known elevation with respect to the plate to measure an actual distance between the backside of the substrate holder and the known elevation while the substrate is planarized.
The distance measuring device may have a beam emitter that projects a source beam, a reflector positioned at a predetermined angle with respect to the plate to direct the source beam against the backside of the substrate holder, and a primary detector to receive a return beam reflected from the backside of the substrate holder normal to the source beam. The primary detector monitors a lateral shift of the return beam and provides a signal to a processor that determines the distance between the backside of the substrate holder and the intersection between the source beam and the return beam. Accordingly, the distance measuring device may measure a first actual distance at one stage of the planarization process and then re-measure a second actual distance at a subsequent stage of the planarization process to determine the change in thickness of the substrate, the polishing rate of the substrate and several other performance parameters of the planarization process while the substrate is planarized.